# Explain why, in a single-slit diffraction experiment, the central maximum is broader and more intense than the secondary maxima.

> IB DP Physics Higher Level (2023 syllabus) — C.3 Wave phenomena · Explain · 4 marks

> A monochromatic light source illuminates a narrow single slit. A diffraction pattern is observed on a distant screen.

## Mark scheme (4 marks)

1. The central maximum forms where path difference between wavelets from across the slit is zero (or less than one wavelength), so all wavelets arrive in phase and interfere constructively.
2. The central maximum spans from the first minimum on one side to the first minimum on the other, giving it an angular width twice that of any secondary maximum.
3. Secondary maxima form where wavelets from the slit only partially cancel, leaving a fraction of the wavelets contributing to constructive interference (approximately one-third, one-fifth, etc. of the slit contributes).
4. Therefore the intensity of the central maximum is much greater than that of secondary maxima because a far greater number of wavelets contribute to its resultant amplitude.

## Key terms

- [single-slit diffraction](https://www.gradenine.co.uk/glossary/single-slit-diffraction)

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-in-a-single-slit-diffraction-278644ef) · Published by Druglandscape Ltd.